Depressurization Method Research Articles

Review of Energy Efficiency of the Gas Production Technologies From Gas Hydrate-Bearing Sediments

Even in the carbon-neutral age, natural gas will be valuable as environment-friendly fuel that can fulfill the gap between the energy demand and supply from the renewable energies. Marine gas hydrates are a potential natural gas source, but gas production from deposits requires additional heat input owing to the endothermic nature of their dissociation. The amount of fuel needed to produce a unit of energy is important to evaluate energy from economic and environmental perspectives. Using the depressurization method, the value of the energy return on investment or invested (EROI) can be increased to more than 100 for the dissociation process and to approximately 10 or more for the project life cycle that is comparable to liquefied natural gas (LNG) import. Gas transportation through an offshore pipeline from the offshore production facility can give higher EROI than floating LNG; however, the latter has an advantage of market accessibility. If the energy conversion from methane to hydrogen or ammonia at the offshore facility and carbon capture and storage (CCS) can be done at the production site, problems of carbon dioxide emission and market accessibility can be solved, and energy consumption for energy conversion and CCS should be counted to estimate the value of the hydrate resources.

An assessment of methane gas production from natural gas hydrates: Challenges, technology and market outlook

Natural gas hydrates are enormous energy resources occurring in the permafrost and under deep ocean sediments. However, the commercial or sustained production of this resource with currently available technology remains a technical, environmental, and economic challenge, albeit a few production tests have been conducted to date. One of the major challenges has been sand production due to the unconsolidated nature of hydrate bearing formations. This review presents progress in methane gas production from natural gas hydrate deposits, specifically addressing the technology, field production and simulation tests, challenges, and the market outlook. Amongst the production techniques, the depressurization method of dissociating natural gas hydrates is widely accepted as the most feasible option and it has been used the most in field test trials and simulation studies. The market for natural gas hydrates looks promising considering the increasing demand for energy globally, limited availability of conventional fossil fuels, and the low carbon footprint when using natural gas compared to liquid and solid fossil fuels. The major market setback currently is cheap gas from shale and conventional oil and gas reservoirs. Cited as: Shaibu, R., Sambo, C., Guo, B., Dudun, A. An assessment of methane gas production from natural gas hydrates: Challenges, technology and market outlook. Advances in Geo-Energy Research, 2021, 5(3): 318-332, doi: 10.46690/ager.2021.03.07

Production Performance Analysis of Class II Hydrate-Bearing Layers Based on an Analytic Aquifer Model

In the current numerical simulation studies, bottom water in Class II hydrate-bearing layers is represented by grids with high water saturation that significantly extends the calculation time if the volume of the bottom water is large or grid size is small. Moreover, the influence of the bottom water volume on the depressurization performance of Class II hydrate-bearing layers has not been fully investigated. In this study, the Fetkovich analytic aquifer model was coupled with a simulation model of a hydrate reservoir to accelerate the simulation of Class II hydrate-bearing layers. Then the simulation results and calculation time were compared between the coupled model and the model in which the bottom water layer is only represented by grids. Finally, the influence of the bottom water volume on the productivity of gas and water in the depressurization method was investigated and the variation of pressure, temperature, and hydrate saturation during the production process was analyzed. The results show that the coupled model can significantly reduce the simulation time of Class II hydrate-bearing layer while ensuring calculation accuracy. When the pore volume of the aquifer increases to 20 times that of the bottom water layer, the computation time of a single model in which the bottom water layer is represented by grids is 18.7 times that of the coupled model. Bottom water invasion slows down the depressurization, and therefore, the larger the aquifer, the lower the peak value of gas production, and the later it appears. However, the invading bottom water can provide heat for hydrate dissociation; therefore, the gas production rate of the hydrate-bearing layer with bottom water is higher than that of the hydrate-bearing layer without bottom water in the late development stage. Generally, the presence of bottom water reduces the cumulative gas production and increases the cumulative water production; therefore, the larger the aquifer, the more unfavorable the depressurization development of the hydrate-bearing layer.

Kinetic study of methane hydrate development involving the role of self-preservation effect in frozen sandy sediments

The dissociation of natural gas hydrate in permafrost areas is a kinetic reaction process associated with the existence of ice and the accompanied self-preservation effect. The kinetic decomposition behaviors of methane hydrate below freezing point are investigated through both experimental and numerical simulations in this work. Dissociation experiments have been conducted in a high-pressure reactor (HPR) by depressurization and wellbore heating methods. Two kinetic models are employed for the prediction of gas recovery and hydrate dissociation in frozen sandy sediments. The modified new model which takes into account the protection effect of ice is found to predict more accurately than the traditional one. When the deposit is situated under frozen state, the hydrate particles can be maintained at a relatively stable state even when the external pressure has dropped below the equilibrium level. Such self-preservation effect is the main factor resulting in the undesirable recovery efficiency of frozen gas hydrate by pure depressurization, and it can be only eliminated by external heat injection. However, the heated wellbore has a limited influencing area, and lots of the provided heat is wasted outside through the boundary. Secondary ice formation exists in the cold regions far away from the well, which leads to incomplete decomposition of methane hydrate. The kinetic decomposition of frozen hydrate is mainly dominated by its self-preservation effect and the injected heat, and the dissociation process is not sensitive to the production pressure when it is adjusted below the equilibrium level.

Design of Ventilation System to Prevent Epoxy Fume Release from Vacuum Pressure Impregnation

In this study, a model for the exhaust of epoxy fumes generated by vacuum pressure impregnation (VPI) was developed using computational fluid dynamics (CFD), and an optimal design for minimizing the fume exhaust time was developed to improve the safety of the working environment. VPI is a process of durability reinforcement by impregnating epoxy varnish insulators to the windings in the stator. In addition, the VPI process can be used for applications such as coating, sealing, and insulating porous parts. The VPI process tanks are composed of a working tank with an inner tank. As the working tank is opened after the vacuum and pressurization to proceed with the final step of the process, the epoxy fumes derived from the resin can leak into the environment. To address these safety and environmental problems, an epoxy fume exhaust process is required. Therefore, a case study was conducted using CFD to validate the exhaust efficiency of epoxy fumes by rearranging different exhaust methods. It was found that the method of using pressurization yielded a higher fume exhaust efficiency (99.46 %) than did the depressurization method (96.42 %).

Microscope insights into gas hydrate formation and dissociation in sediments by using microfluidics

Natural gas hydrates (NGHs) have tremendous potential and abundant reserves worldwide. Both the hydrate distribution and the potential inverse formation are related to the efficient exploitation of NGHs; however, studies of the micro-mechanism of hydrate morphology and the evolution of phase transition processes are still lacking. In this study, hydrate formation and dissociation were investigated at the microscale using a microfluidics device. Methane hydrate (MH) was formed at a system pressure of 5 MPa and temperature of 274.15 K and then was dissociated by using the depressurization method. Based on different gas–water contact areas, two kinds of stable crystal structures and two kinds of unstable crystal structures of the micromorphology in the formation stage were identified. During the dissociation process, the direct proof of the induced local re-formation of the hydrate by microbubble aggregation was given. First, the distribution of the CH4 bubbles (from 5 μm to 140 μm) inside the pores and throats was quantified. Normal bubble distributions were found, and the largest percentage of bubble diameters ranged from 10 μm to 30 μm. The average diameter of the bubbles increased with time, and the total number of bubbles decreased with time. The large density distribution of microbubbles with smaller diameters in liquids impeded the pressure propagation and heat transfer, which are keys to restraining the rate of hydrate dissociation. These findings are beneficial for understanding the microscale mechanisms of the hydrate phase transition and MH dissociation efficiency, which may be helpful for the selection and design of NGH exploitation schemes.

Enhancing gas production from Class II hydrate deposits through depressurization combined with low-frequency electric heating under dual horizontal wells

Class II hydrate deposits are characterized by a mobile water zone (WZ) underneath the hydrate-bearing layer (HBL) and are encountered in permafrost and deep-sea sediments. In this work, an efficient method of depressurization combining low-frequency electric heating under dual horizontal wells is proposed to exploit Class II hydrate deposits, in which two parallel horizontal wells are arranged in the HBL and the WZ. Based on the geological parameters in the Mallik deposit, the energy recovery behaviors are studied through a numerical simulation approach. Electric heating significantly improves hydrate dissociation and gas production compared with the depressurization method. However, gas production lags electric heating for a long time, and the energy efficiency ratio decreases with time in the later stage. To address these shortcomings, two additional electric heating schemes are designed and optimized. The results show that the additional wellbore heating at the beginning of production accelerates the dissociation of hydrates near the production well, thus greatly reducing the lag time. When electric heating is terminated after the 800th day, the cumulative gas production is reduced by 9.1%, but the energy efficiency ratio is improved as high as 48.71, which confirms the great potential of the proposed method.

Numerical Study of Gas Production from a Methane Hydrate Reservoir Using Depressurization with Multi‐wells

AbstractWith the implementation of the production tests in permafrost and offshore regions in Canada, US, Japan, and China, the study of natural gas hydrate has progressed into the stage of technology development for industrial exploitation. The depressurization method is considered as a better strategy to produce gas from hydrate reservoirs based on production tests and laboratory experiments. Multi‐well production is proposed to improve gas production efficiency, to meet the requirement for industrial production. For evaluating the applicability of multi‐well production to hydrate exploitation, a 2D model is established, with numerical simulations of the performance of the multi‐well pattern carried out. To understand the dissociation behavior of gas hydrate, the pressure and temperature distributions in the hydrate reservoir are specified, and the change in permeability of reservoir sediments is investigated. The results obtained indicate that multi‐well production can improve the well connectivity, accelerate hydrate dissociation, enhance gas production rate and reduce water production as compared with single‐well production.

Enhanced gas production from marine hydrate reservoirs by hydraulic fracturing assisted with sealing burdens

Gas productivity of current hydrate reservoir tests is too lower to meet the demand of the commercial level. The main reasons include the low permeability of hydrate-bearing layer (HBL) and the existence of permeable burdens in natural gas hydrate reservoir, which act as the major barriers for pressure drop propagation during depressurization. However, previous studies are focusing on either permeable burdens or hydraulic fracturing. This paper proposed a novel modified method of depressurization by hydraulic fracturing assisted with sealing burdens. The effects of the radial length and permeability of fractured domain, and the sealing location and length of overburden and underburden layers on hydrate dissociation and gas production were analyzed through numerical simulation. The results indicated:1) The larger permeability and longer radial length of the fracturing domain would promote propagation of pressure drop significantly, and the percentage of gas hydrate dissociation would be increased from 3.99% to 29.86% by hydraulic fracturing. 2) The sealing length of burdens should be larger than the fracturing radial length. The cumulative gas production could be enhanced by 93.25% while the cumulative water production could be decreased by 62.99% compared with no sealing burdens. Base on the novel method, the goals of enhancing gas production and reducing water production would be achieved simultaneously, providing important implications for hydrate commercial exploitation in the future.

An exergy-based energy efficiency analysis on gas production from gas hydrates reservoir by brine stimulation combined depressurization method

Before performing commercial scale production of natural gas hydrates (NGHs), it is inevitable to estimate the productivity, production period, optimal operation parameters, and further evaluate the technical and economic feasibilities of NGHs field production. The dissociation of gas hydrates is an endothermic process. This needs to compensate additional energy, such as chemical energy, geothermal energy and electricity, etc., in order to keep high dissociation rate. Exergy is a basic parameter that used for various energy systems as a potential tool for design, analysis and optimization. This work provided an exergy analysis method to evaluate the economic efficiency of NGHs exploitation process by brine stimulation combined depressurization method. The input data including cumulative volume of gas production, mass of water production and gas-to-water ratio was obtained by the numerical simulation of TOUGH + HYDRATE. Through the exergy analysis, the total productivity and production period could be determined. These have rarely been reported. The influences of operation parameters, such as well spacing, injection temperature and injection rate of brine on net exergy production were investigated for guiding the set of optimal operating conditions. Since the cost may be one of the biggest challenges for NGHs production, this work suggests making full use of all kinds low-level energy sources to improve energy efficiency, such as brine with high concentration produced by desalination, tidal energy, wind energy, etc.

The Characteristics of Gas-Water Two-Phase Radial Flow in Clay-Silt Sediment and Effects on Hydrate Production

Many hydrate-bearing sediments in the Shenhu area of the South China Sea are featured with unconsolidated clayed silt, small particle size, and high content of clay, which can pose a great challenge for gas production. In order to investigate the gas-water relative permeability in clay-silt sediments, through a radial flow experiment, samples from the target sediment in the Shenhu area were selected and studied. The results show that the irreducible water saturation is high and the influence of the gas-water interaction is obvious. The relative permeability analysis shows that the two-phase flow zone is narrow and maximum gas relative permeability is below 0.1. The flow pattern in clay-silt sediment is more complicated, and the existing empirical models are inadequate for flow characterization. The depressurization method to extract a hydrate reservoir with clay-silt sediments faces the problem of insufficient production capacity. Compared with the ordinary hydrate reservoir with sandstone sediment, the hydrate reservoir with clay-silt sediment has a low permeability and poor gas flow capacity. The gas-water ratio abnormally decreases during the production. It is urgent to enhance production with cost-effective measures.

AI-based composition model for energy utilization efficiency optimization of gas hydrate recovery by combined method of depressurization and thermal stimulation

The combination of depressurization and thermal stimulation is one of the most promising techniques for producing gas from natural gas hydrate reservoirs. The energy utilization efficiency is the core factor that determines whether the technology can be better applied to engineering practice. In contrast to previous works, this paper proposes an AI-based composition model, which realizes the dynamic optimization of the energy utilization efficiency based on the learning and evaluation of different thermal stimulation policies. As shown by the optimization results, the proposed model achieved good performance in both the hydrate recovery target and the economic target coordination by adjusting the coefficients of the optimization control policy function. Based on a systematic assessment of the energy utilization efficiency of different thermal stimulation policies, the model increased the energy production-injection ratio by 4.5 times in Scenario I, and the CH4 recovery efficiency was increased by 1.6 times in Scenario II. The learning results of the AI model also showed that the reactor scale hydrate recovery had a size effect, such that the later the thermal energy injection was, the higher the energy efficiency that could be obtained. Moreover, through the model learning, the quantitative relationship between each thermal stimulation policy and its developmentally changing energy utilization efficiency was established. The results showed that in the initial hydrate recovery period, thermal stimulation policies with a high injection temperature and low injection rate achieved greater energy utilization efficiency. As recovery progressed, gradually lowering the injection temperature and increasing the injection rate gave the policy a higher energy utilization efficiency. The proposed methodology provides a feasible approach for dynamic thermal stimulation parameter optimization and improves the hydrate recovery economy.

Concurrent decomposition and replacement of marine gas hydrate with the injection of CO2-N2

• CO 2 -N 2 was injected into gas hydrate sediments under real reservoir conditions. • Concurrent CH 4 hydrate decomposition and CO 2 -N 2 replacement occurs distinctly. • Local non-equilibrium with a higher CH 4 concentration leads to a local replacement. • Both of decomposition and replacement contributes to gas recovery. • Dual production pattern combines advantages of depressurization and replacement. CO 2 replacement has been regarded as a promising method for the recovery of gas hydrate. The kinetics and feasibility of its application to gas hydrate reservoir are extensively studied at low temperatures that are proper for the formation of CO 2 /N 2 hydrates. However, in facts, most of the gas hydrate reservoirs in marine sediments have relatively high temperature conditions that are below the hydrate equilibrium line of CO 2 /N 2 . In this work, CO 2 -N 2 gas was injected into gas hydrate sediments under the pressure–temperature conditions of real marine hydrate reservoirs to evaluate the kinetic behavior of the CO 2 -N 2 replacement at relatively high temperatures. It was observed that the CH 4 production was more prominent than the sequestration of CO 2 -N 2 . The results indicate that concurrent decomposition and replacement occur distinctly in gas hydrate reservoir with relatively high temperatures after the injection of CO 2 -N 2 . It is proposed that the local non-equilibrium with a higher concentration of CH 4 near the dissociating hydrate surface than that of the bulk gas phase can lead to a local CO 2 -N 2 replacement. The contribution of decomposition and replacement to gas recovery varies with the temperature and pressure conditions. Higher temperature and lower pressure would promote the decomposition and inhibit replacement, enhancing the gas recovery efficiency. This dual production pattern combines the advantages of both depressurization and replacement, which would substantially improve the safety of depressurization method for weak hydrate reservoir conditions in efficient gas recovery as the geologic storage of CO 2 -N 2 in hydrate form consolidates the sediments. These findings extend the previous knowledge on CO 2 replacement method and provide insights for a complete understanding of the application of this method to the real marine gas hydrate reservoirs.

Numerical simulation on gas production from inclined layered methane hydrate reservoirs in the Nankai Trough: A case study

Natural gas hydrate is generally disseminated in tilted sediments. The inclination of hydrate reservoirs significantly affects the fluid migration behaviour and production potential. However, most of the current simulations ideally assume that the hydrate reservoir is horizontally distributed; resulting in the effect of the formation dip on the well deployment and corresponding gas production remains unclear. Here, we implement a real inclined alternating sand–clay hydrate reservoir model based on geological data from the Nankai Trough, Japan, to simulate the production performance of a single horizontal well and dual horizontal wells. Results show that the gas and water productions are enhanced when a single horizontal production well is deployed in the structural low of the reservoir. The gas and water production performance in upper sandy reservoir and the water extraction in the middle muddy reservoir are sensitive to the layout of the production wells, which can cause the daily difference is over 6%. The gas-to-water ratio is independent from the structural deployment positions of the horizontal wells in the same inclined layer. Compared to the single depressurization method, placing a horizontal thermal injection well in the structural high of the upper inclined sandy reservoir and a horizontal production well in the structural low of the same formation improves the gas recovery efficiency nearly twice, which is recommended for production enhancement. Our findings are useful for achieving effective and economic gas recovery from natural inclined layered hydrate reservoirs.

Study on the Influence of Sand Production on Seepage Capacity in Natural Gas Hydrate Reservoirs

Sand production has become a common phenomenon in the exploitation of unconsolidated natural gas hydrate reservoirs, which will hinder the long-term production of natural gas hydrate reservoirs. However, there are few literatures reported on the influences in reservoir physical properties such as permeability and porosity, and production laws caused by sand production. This paper provides a numerical model, coupled with reservoir sand-gas-water multiphase flow processes, which is capable to simulate the process of sand production in natural gas hydrate reservoirs. The simulation results indicate that sand settlement is mainly concentrated near the wellbore due to the high concentration of migrated sand. The decrease in reservoir porosity and permeability caused by sand settlement has a significant impact on production. The impact of sand production on reservoir fluid fluidity shows that fluid flow is inhibited near the wellbore, while fluid flow performance increases far away from the wellbore. The numerical model and analysis presented here could provide useful insight into changes in reservoir physical properties and production laws caused by sand production in the natural gas hydrate-bearing marine sediments using depressurization method.

Hydrate blockage observation and removal using depressurization in a fully visual flow loop

Hydrate blockages impose a serious flow assurance problem on gas transportation pipelines. Regular inspections for the formation of hydrate blockages are imperative along with the use of an effective blockage removal method. In this study, the processes of hydrate blockage formation and removal were investigated using a fully visual flow loop. Comprehensive analysis of the pressure drop was performed along with a discussion of the acquired visual images of the hydrates to describe the phenomena. Hydrate blockage first occurred in no through-flow zone of the pipeline, specifically in deadleg. The results showed that higher LL (liquid loading: the initial liquid volume fraction) caused a blockage in the inlet, while lower LL caused a faster and more widespread blockage. Moreover, shut-in and restart operations also caused blockages due to the temperature decrease and annealing of hydrate. The experiments showed that blockages formed more quickly as a result of lower LL during the shut-in operation. This necessitated the reduction of shut-in duration and maintenance of the temperature at phase equilibrium condition. In addition, the depressurization method was used to decompose bulk hydrate in order to remove the blockage. The results confirmed that stepwise depressurization could be applied to prevent hydrate reformation at a higher LL and water conversion rate, while a lower backpressure could be used at lower LL to efficiently remove blockages.

Experimental study on in situ dissociation kinetics of CO2 hydrate in pure water and water/sediments systems

AbstractRecently, hydrate technology as a newly emerging field has been attracting more and more attention. To support its potential applications, the dissociation behaviors and kinetics of CO2 hydrate in pure water and water/sediments systems at specified temperatures were studied experimentally by depressurization method. This work reveals two novel aspects of CO2 hydrate dissociation. Firstly, it is remarkable that the dissociation rate of CO2 hydrate in water/sediments is faster than that of CO2 hydrate in pure water, which has not been conscious previously. Secondly, a pseudo first‐order kinetic equation including the kinetic constant and activation energy was formulated to describe the dissociation process. Temperature plays an important role and the dissociation rate constant () and activation energy () were obtained through the dissociation experiments at different temperatures. For CO2 hydrate in pure water, the dissociation rate constant increased from 0.02 to 0.13 mol/(dm2⋅MPa⋅min) at the temperature from 273.86 to 276.11 K, and the activation energy was 469.06 kJ mol−1. For CO2 hydrate in water/sediments, the dissociation rate constant was from 0.03 to 0.15 mol/(dm2⋅MPa⋅min) at the temperature ranging from 273.45 to 276.11 K, and the activation energy was 346.30 kJ mol−1. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd.

Investigating the Interaction Effects between Reservoir Deformation and Hydrate Dissociation in Hydrate-Bearing Sediment by Depressurization Method

Natural gas hydrate (NGH) has been widely focused on having great potential for alternative energy. Numerous studies on gas production from hydrate-bearing sediments have been conducted in both laboratory and field. Since the strength of hydrate-bearing sediments depends on the saturation of NGH, the decomposition of NGH may cause the failure of sediments, then leading to reservoir deformation and other geological hazards. Plenty of research has shown that the reservoir deformation caused by hydrate decomposition is considerable. In order to investigate this, the influence of sediment deformation on the production of NGH, a fully coupled thermo-hydro-chemo-mechanical (THMC) model is established in this study. The interaction effects between reservoir deformation and hydrate dissociation are discussed by comparing the simulation results of the mechanical coupling and uncoupled models on the laboratory scale. Results show that obvious differences in behaviors between gas and water production are observed among these two models. Compared to the mechanical uncoupled model, the mechanical coupling model shows a significant compaction process when given a load equal to the initial pore pressure, which leads to a remarkable decrease of effective porosity and reservoir permeability, then delays the pore pressure drop rate and reduces the maximum gas production rate. It takes a longer time for gas production in the mechanical coupling model. Since the reservoir temperature is impacted by the comprehensive effects of the heat transfer from the boundary and the heat consumption of hydrate decomposition, the reduced maximum gas production rate and extended gas production process for the mechanical coupling model lead to the minimum reservoir temperature in the mechanical coupling model larger than that of the mechanical uncoupled model. The reduction of the effective porosity for the mechanical coupling model causes a larger cumulative water production. The results of this paper indicate that the reservoir deformation in the gas production process should be taken into account by laboratory and numerical methods to accurately predict the behaviors of gas production on the field scale.

Numerical modeling of gas production from methane hydrate deposits using low-frequency electrical heating assisted depressurization method

With the increasing global demand for clean energy, methane hydrate has been regarded as a potential alternative energy source in future due to its huge resources and high energy density. In this study, a mathematical model is established to evaluate the production performance of low-frequency electrical heating assisted depressurization (LF-EHAD) method for hydrate deposits recovery. Within the framework of this model, the integral finite difference method and Newton-Raphson iteration are employed for solution, which is then validated by the experiment data. Using this model, the energy recovery behaviors with LF-EHAD from Shenhu hydrate deposits are investigated and then compared with those performed by hot water flooding and depressurization. Simulation results show that hydrate dissociation and gas production are significantly enhanced as a result of sufficient heat supplied by the electrical heating, validating the potential of the LF-EHAD for hydrate deposits recovery. After 2000 days of production, the cumulative gas production of LF-EHAD and hot water injection are 2.37 times and 1.47 times of that conducted by the depressurization method. Moreover, the heat consumption used to increase the formation temperature in the hydrate dissociation region under LF-EHAD is much less than that of hot water flooding, which results in a higher energy efficiency ratio (EER) of 9.65 for LF-EHAD, while the EER of hot water flooding is 4.42. Overall, LF-EHAD shows better performance than hot water flooding with the same amount of heat input because of higher gas production and energy utilization efficiency, but lower water production.

Comprehensive Evaluation of Gas Production Efficiency and Reservoir Stability of Horizontal Well with Different Depressurization Methods in Low Permeability Hydrate Reservoir

Comprehensive Evaluation of Gas Production Efficiency and Reservoir Stability of Horizontal Well with Different Depressurization Methods in Low Permeability Hydrate Reservoir